Generally, the load for a power converter has diversified forms. For example, the load includes a resistive impedance, an inductive impedance, a capacitive impedance, or a combination thereof. When the input current to the load and the input voltage to load are in phase, the power factor is close to 1. If the power factor is lower than 1, the transmitted power possibly loses because of phase mismatch between the current and the voltage or noise.
For avoiding the power factor reduction and increasing the power transmission efficiency, the conventional power converter has a power factor correction function. For example, the conventional power converter is usually equipped with an active type power factor correction circuit to achieve the power factor correction function. The power factor correction circuit is controlled in a feedforward control manner. That is, the power factor correction circuit samples the AC input voltage and adjusts the output current according to the AC input voltage. Consequently, the AC input current is changed with the AC input voltage. In such way, the AC input current with a nearly sine wave and identical phase is obtained. Consequently, the power factor is increased, and the total harmonic distortion is increased.
However, the conventional power factor correction circuit still has some drawbacks. For example, the conventional power factor correction circuit usually comprises a bridge rectifier diode and a high frequency filtering capacitor. Since the bridge rectifier diode has forward voltage drop and the high frequency filtering capacitor is located downstream of the bridge rectifier diode, the AC input current may cease and distort in the vicinity of the zero-crossing point of the AC output voltage. Under this circumstance, the zero-crossing distortion occurs. Consequently, the total harmonic distortion increases, and the power factor decreases.
Therefore, there is a need of providing an improved phase compensation method for a power factor correction circuit in order to overcome the above drawbacks.